1. Field of the Invention
This invention relates generally to flow detection systems, and more specifically to a sensor-based device for detecting a liquid-to-gas or gas-to-liquid phase change of a flowing fluid, in addition to measuring the temperature of the fluid and sensing variations in the flow rate of the fluid.
2. Description of the Related Art
Many processes use a pumping system to transfer a liquid from one location to another. In general, a typical pumping system consists of a motor, a pump, a mechanism to couple the motor to the pump, and at least a simple controller such as an ON/OFF switch. Most pump designs require that the internal parts of the pump be completely wet, or xe2x80x9cprimed,xe2x80x9d and that liquid flow rates remain above some minimum threshold. These requirements usually ensure adequate lubrication and cooling of the pump parts. Loss of prime, or flow rate reductions below the minimum threshold, which may be due to cavitation, conduit leakage or blockage, can cause detrimental over-heating and premature failure of the pump as well as other parts of the pumping system. The size of the pumping system is usually directly related to its thermal failure acceleration factor and repair cost. In other words, the larger the pumping system, the quicker it might fail if deprived of sufficient liquid flow, and the more expensive it could be to repair or replace the damaged parts.
The artificial lift of crude oil from underground formations and the filling or emptying of liquid storage vessels or tank cars are common examples that rely upon the use of relatively large, costly pumping systems. In both cases, the ability to promptly detect a liquid-to-gas or gas-to-liquid phase change of the flowing fluid is crucial to the proper operation of the system. Prompt phase change detection is also critical for maintaining safe, efficient, and reliable pumping system operation. Furthermore, being able to determine whether the fluid flow rate is increasing or decreasing, particularly detecting complete liquid or gas stoppage, which could be due to blockage of the conduit, provides greater ability for process management and pump control.
Numerous attempts at providing protection and effective control for pumps have been made for the application examples cited above as well as others, but with significant limitations in their levels of success. Sensors or combinations of sensors have been used to measure the electrical or mechanical energy being delivered to the pump; to detect the presence or absence of liquid at the pump; and to monitor the liquid temperature, pressure and flow rate through the pump. The energy measuring methods are indirect, or inferred measurements and, consequently, can be inaccurate and unreliable. Among the other methods used, though some are direct, most are single parameter measurements. Single parameter measurements typically have not been sufficiently sensitive, fast or smart enough to detect the combination of effects that indicate the valid occurrence of a liquid-to-gas or a gas-to-liquid phase change.
There is a recognized need in many situations where fluids are moved or transferred, for a rugged, industrial device that can precisely detect a liquid-to-gas or a gas-to-liquid phase change while the fluid is flowing through a conduit such as a pipe.
A major purpose of the present invention is to provide a simple and effective means to precisely determine fluid flow characteristics by employing a combination of a flow conditioner, a sensor, and a controller. The liquid/gas phase detector system of the preferred embodiment of the invention has the purpose and ability to identify and monitor the flow of either a liquid or a gas, or a combination, through a conduit such as a pipe. In addition, the invention detects the phase of the flowing fluid, stoppage of flow of either gas or liquid, flow surges, changes in the fluid flow rate through the conduit, and even the fluid temperature.
In a preferred embodiment of the invention, the flow conditioner or pipe segment is preferably a relatively short length of pipe having a reverse bend or an inverted V-shaped section forming a dam, or weir, over which the liquid flows. This pipe segment is coupled in a horizontal conduit, usually downstream of the pumping system and upstream of a check valve (if one is used). The inlet and outlet ends of the pipe segment preferably are axially aligned, with the top, or crest, of the weir being above the centerline or axis of the inlet of the pipe segment, but below the top dead center of the inside surface of the inlet. The inside diameter of the pipe segment is substantially constant from one end through to the other. During normal operation a reservoir of liquid forms on the upstream side of the weir. A reference plane is defined by the surface of the reservoir when the liquid is at the crest of the weir. The reference plane is therefore above the centerline of the inlet and outlet ends of the pipe segment. Depending on the conduit configuration upstream of the pipe segment, the surface area of this reference plane and hence, the upstream ullage, could be relatively large. Even with the reservoir filled to the level of the reference plane, this upstream ullage allows gas to flow through the pipe segment.
Some applications might require that the height of the weir crest be slightly above the top dead center of the inside surface of the inlet. This would form a liquid seal or gas trap, and prevent gas from flowing through the pipe segment if there is stoppage of liquid flow while the reservoir remains filled to the level of the reference plane. Upstream ullage is thereby zero due to this gas trap and the surface area of the reference plane is small (approximately equal to xcfx80/4D2, where D is the pipe segment inside diameter at that level).
A sensor is mounted in the pipe segment wall on the upstream side between the weir and the inlet end. The preferred sensor utilizes thermal dispersion technology. This sensor has the ability to detect the presence or absence of liquid flowing over the weir, and to monitor the variation of the flow rate of gas or liquid through the pipe segment. It can also provide the temperature and phase of the flowing fluid, that is, whether there is liquid or gas flowing.
The controller converts the raw output signal from the sensor into useful control signals and display values. In its simplest form, the controller consists of a few interconnected functional blocks. The major blocks, in addition to a power supply, are input and output signal conditioners, and a signal processor with display, input keypad, and a memory. The memory is used with the processor in order to store and retrieve the operational instructions of the controller as well as the factory and end-user setup and calibration parameters. The signal processor contains timers and counters that are used for the timing, accumulation, and sequencing of input and output events. Relay contacts, analog voltages or currents, status lamps, visual displays, digital interfaces, audio signals, or any combination thereof can be configured as the outputs of the controller.
Once the pipe segment is filled with liquid, additional liquid flow causes the reservoir to rise above the reference plane and liquid to flow over the weir. The sensor is positioned just above the reference plane and detects a gas-to-liquid phase change when wetted by the rising reservoir level. As increased liquid flow rates further raise the reservoir level, the sensor monitors the changing liquid flow rate. The height (or depth) of liquid above the crest of the weir is relative to the liquid flow rate. When liquid flow stops, the reservoir level quickly returns to the level of the reference plane. At that point the sensor is no longer wetted and it detects a liquid-to-gas phase change. A typical structure of the flow conditioner and sensor of the system is such that if gas is flowing through the pipe segment, the gas flow can be monitored even though there is no liquid flow.
The sensor of the system has a fast response time and can monitor very low gas or liquid flow rates. This sensitivity enables the sensor to detect flow surges caused by entrained gas xe2x80x9cslugs.xe2x80x9d Further, the sensor has the ability to detect gas pressure changes.
A particular feature of the phase detector system of the invention is that mechanical dependability is assured because there are no moving fluid-wetted parts or orifices to fail or foul. Another feature is a high signal-to-noise ratio which enables clear indications of phase change, flow rate, and fluid temperatures, among others.
Alternative embodiments of the flow conditioner are disclosed for modified flow detection purposes. The principle of operation of all the flow conditioner embodiments is the same.